This invention relates to an air-gap optical structure and, more particularly, to such an air-gap optical structure wherein the optical elements are spaced apart by a layered metallic spacer structure.
Air-gap optical structures are widely used in optical systems. In an example, two prisms separated by an air gap are used in some designs of reflective digital micromirror device (DMD) projectors. The air-gap optical structure provides for total internal reflection at an air-gap face for some incident angles of light, but transmission through the air-gap face for other incident angles of light. In the projector, an incident beam is reflected from the air-gap face to the reflective DMD display under conditions of total internal reflection, reflected from the DMD display according to the image thereon and back toward the air-gap face, and passed through the air-gap face under transmission conditions for viewing.
The air-gap optical structure is formed by providing two transmissive optical elements (such as prisms or lenses) of the proper shapes, and placing them together in a facing relationship with the air gap defined between the adjacent faces. A number of techniques have been used to form the air gap. For example, in various instances metal shims, plastic tapes, and spacer balls have been placed between the adjacent faces to define the air gap. These approaches are all operable to some extent, but have shortcomings in that the position of the spacer may shift or the thickness of the spacer may change over time and under various service conditions. Shims and similar spacer devices are difficult to work with in thicknesses that define air-gap spacings of 20 micrometers or more. They become nearly impossible to work with in thicknesses that define air-gap spacings of about 5-15 micrometers, as is required for some applications. Additionally, they are not available in such small dimensions in many cases, and in other cases are available from manufacturers only in specific dimensions selected by the manufacturers.
There is consequently a need for an improved approach to the fabrication of air-gap optical structures. The present invention fulfills this need, and further provides related advantages.
The present invention provides an air-gap optical structure in which the air gap is defined by a spacer structure whose position and thickness are precisely defined. Neither the position nor the thickness of the spacer structure changes over time and with use, and under the normal conditions experienced in service. The present approach is readily implemented in production-scale operations at a reasonable cost. The present approach is preferably implemented in an air-gap prism structure, but it may be used in other applications such as air-gap lens doublets and Phillips color prisms, for example.
In accordance with the invention, an air-gap optical structure comprises a first transmissive optical element, a second transmissive optical element, and a spacer structure having a spacer thickness and disposed between the first transmissive optical element and the second transmissive optical element so as to define an air gap therebetween having the spacer thickness. The spacer structure comprises a metallic structure having a first layer made of a first material contacting the first transmissive optical element, and a second layer made of a second metal contacting the second transmissive optical element. The first material is preferably the metal iron, but it may be any operable material, and the second metal is preferably nickel.
The spacer thickness is preferably from about 5 to about 15 micrometers, but it may be larger or smaller. The first layer typically has a first-layer thickness of less than about 1 micrometer, and the second layer typically has a second-layer thickness of the balance of the spacer thickness, and normally from about 5 to about 15 micrometers. The first layer preferably has a vapor deposited microstructure, and the second layer preferably has an electroless-deposited microstructure. The shape of the spacer structure may be of any desired form, but its preferably comprises at least three, and most preferably exactly four, pads.
A method for fabricating an air-gap optical structure comprises the steps of furnishing a first transmissive optical element, furnishing a second transmissive optical element, depositing a spacer structure on the first transmissive optical element, and contacting the second transmissive optical element to the spacer structure so that the spacer structure lies between the first transmissive optical element and the second transmissive optical element and defines an air gap therebetween. The step of depositing the spacer structure includes the steps of depositing a first layer made of a first material onto the first transmissive optical element, and depositing a second layer made of a second metal onto the first layer. The first layer is preferably deposited by vapor deposition, and the second layer is preferably electroless-deposited upon the first layer. The thickness are preferably as set forth previously.
The present approach results in an air-gap optical structure wherein the transmissive optical elements are spaced apart by the metallic spacer structure having two or more metallic layers. The elements of the spacer structure are deposited in place upon the first transmissive optical element, and consequently their positions cannot later shift during service. The spacer structure is metallic and preferably made of metals that do not substantially deform under the normal service conditions of the air-gap optical structure, so that the thickness of the air gap does not change over time and in service. The air-gap optical structure is therefore highly stable. The metallic layers are deposited by techniques that define their thicknesses precisely, so that the thickness of the air gap is defined precisely and there is no optical wedge effect resulting from variations in the thicknesses of the various portions of the spacer structure. By contrast, techniques that use shims or spacer balls to define the air gap usually result in the undesirable optical wedge effect because their thicknesses cannot be controlled to the same degree of precision as can the spacer structure of the present approach. The present approach also results in spacer thicknesses that are very precisely defined and may be selected for particular applications. On the other hand, shims and spacer balls are usually available only in sizes selected by their manufacturers, which may not be suitable for particular optical-gap applications.